. The goal of the proposed research is the identification of improved agents for the treatment of toxoplasmic encephalitis and Pneumocystis (P) carinii pneumonia which result from opportunistic infections in patients with AIDS and other immunosuppressive disorders. Specifically, agents that inhibit the function of dihydrofolate reductase (DHFR), an enzyme vital to Toxoplasma (T) gondii and P. carinii, will be identified. Another aim is to design more effective and selective lipophilic antifolates using structure- activity relationships gained from a current project (U01 AI30279, expiration date June 30, 1995) as well as crystal structure-based methods. In their current work, these investigators have identified several agents active against both T. gondii and P. carinii DHFR, some of which are also active against T. gondii in culture, and even though they are analogous with non-selective trimetrexate and piritrexim, these compounds have significant selectivity toward the pathogenic DHFRs over mammalian DHFR. The active and selective lipophilic antifolates feature a ,4-diamino- 5-methyl -5-deazapteridin-6-yl group attached through a two-atom bridge (-CH2NR-, -CH2S-, -CH2CH2-) to a side chain conferring lipophilicity. The investigators now propose to combine the best features of the side chains and bridges to produce improved agents while simultaneously determining the crystal structures of some of their current inhibitors complexed to human and P. carinii DHFR. Information gained from these comparative structural studies will be incorporated into the design of more selective and potent lipophilic antifolates against P. carinii. Preliminary crystallographic results have also suggested three different modifications of trimethoprim which would be expected to increase its selectivity toward P. carinii DHFR. To test these hypotheses, trimethoprim analogues will be synthesized for evaluation, and their crystal structures will be determined. Antifolate candidates synthesized under this project will be evaluated for efficacy by Dr. Sherry F. Queener of Indiana University Medical School under a subcontractural agreement and also by other laboratories supported by contracts with NIAID. The compounds will be tested in a two-part primary evaluation for (a) their inhibitory effect against DHFR from both T. gondii and P. carinii and also from a mammalian source (rat liver), and (b) their ability to inhibit growth of the pathogenic organisms in culture. Agents whose primary screening results show them to warrant further study will be tested in mice infected with both T. gondii and P. carinii.